NMR imaging processes and respective devices are used, i.a., for non-destructive examination of the material and the structure of objects composed of crystalline, glass-like materials or of soft materials, such as for example elastomers, or of liquids or biological materials.
A prior art NMR process and a respective device is disclosed in DE 199 39 626 A1, in which a portable NMR device is described with reference to FIGS. 3a and 3b. This prior art device comprises two permanent magnets which are spaced a distance apart and serve to generate a stationary magnetic polarization field. A high-frequency coil arrangement and two gradient coils are provided between the permanent magnets. The two gradient coils serve to generate a magnetic gradient field superposing on the stationary magnetic polarization field. The magnetic gradient field is operated an intermittently pulsed manner for locally resolved NMR measurement.
With the aid of the high-frequency coil arrangement, which, in addition to generating the high-frequency alternating field, simultaneously acts as a reception coil, high-frequency signals generated by the nuclear resonant processes occurring inside the sample, are received and visually represented by means of an evaluation unit. The nuclear resonant processes occurring inside the sample are predominantly due to the nuclear spin interaction of the hydrogen atoms with the stationary magnetic polarization field based on the selective high-frequency energy excitation by the high-frequency alternating field and can be put in a very remote sense in relation to the hydrogen concentration present inside the examined sample volume in the evaluation.
Although the NMR device described in the preceding when built as a portable device permits planar examination of an object by running over the surface of the object, the to-be-analyzed object volume is limited to only a very shallow depth of penetration of a few millimeters due to the magnetic fields generated by the NMR device. With certainty the penetration depth into the to-be-examined object could be improved by dimensioning the permanent magnets employed correspondingly larger. However, such scaling measures influence the actual magnetic depth of penetration only insignificantly. Moreover, the larger and thus heavier permanent magnets make the NMR device impossible to use as a portable device.
Furthermore, for locally resolved NMR analysis, the NMR system described in the preceding requires gradient coils to generate magnetic gradient fields, which requires complicated activation, thus entailing technically complicated test-signal inducement.
In order to reduce the complexity of the apparatus of the NMR system, DE 36 39 140 A1 proposes reducing the volume region of the stationary magnetic polarization field due to which, however, the magnetic field does not penetrate the entire to-be-examined patient but only a section. The stationary magnetic polarization field is generated with the aid of a Helmholtz double magnetic system, leading to tapered bundling of the magnetic field lines with increased magnetic flux density. The to-be-examined patient is positioned in this tapered region of the magnetic field lines. In order to examine the whole patient, the patient is placed in a moveable manner in relation to the magnetic system. In this case, too, the magnetic resonance system described in this printed publication provides a gradient coil arrangement for local resolution of the measuring points within the tapered region of the magnetic field lines.
EP 0 228 154 A2 describes a magnetic arrangement comparable to the preceding magnetic resonance system. This magnetic arrangement shows a double magnetic arrangement suited for NMR purposes. In order to reduce magnetic losses due to so-called magnetic field leakage, the magnetic arrangements are each provided with a concave-designed top side facing each other, which also makes the magnetic lines assume a tapered field shape between the magnetic arrangements. In this instance, however, additional gradient fields are required in order to permit local resolution when examining samples.